The invention relates generally to combined encoder/decoder circuit arrangements and more particularly, to an improved circuit arrangement of this type wherein a frequency selective resonant reed is used in an oscillator circuit having both positive and negative feedback.
In many electronic applications it is necessary to identify when a received signal has a tone which exactly corresponds to a precise predetermined frequency. Such signal identification is commonly referred to as decoding. One typical example of a coding system application is a radio personal paging system in which a tone, or sequence of tones, is used to alert one of many paging receivers.
The identification and the generation of precise frequency tones may be effected by using a resonant reed which presents a low impedance to signals having frequencies that substantially correspond to the resonant frequency of the reed. A high impedance is presented to signals having other frequencies. Normally a single resonant reed is used in a decode circuit to identify a specific tone and an additional separate resonant reed is used in an oscillator circuit to generate a specific tone which is used for encoding a signal.
Because resonant reeds are costly, some prior systems use a single reed in a combined encoder and decoder circuit in which the circuit is selectively switch between either an encoding or decoding mode of operation. In such circuits, an encoding signal is produced by an oscillator which includes the resonant reed and the operation of the oscillator is terminated when a decoding function is desired. Such systems are not capable of providing simultaneous encode and decode operation and require complex switching circuits.
The frequency selective element (resonant reed) normally has a very slow initial response time and a slow turn off time. This is because the frequency responsive element is a high Q circuit which stores energy. Thus the element has a slow start up time, required to build up the necessary stored energy, and a slow turn off time, required to dissipate the stored energy. These properties of the reed create many design problems for the prior art selectively switched combined encoder and decoder circuit. The problems result because of the need for an instantaneous encoding signal when encoding operation is desired and the need for a decoder circuit which rapidly and properly responds to a desired input signal.
The decode response times of the prior art combined encoder/decoders are primarily determined by the Q of the frequency responsive element. Since high Q elements are required for efficient frequency selection, prior art circuits have had slow initial response times. Additionally, the slow turn off time of the resonant reed has resulted in prior circuits being especially susceptible to high energy pulses creating a false tone detection. Such "falsing" is typically caused by a high energy pulse shocking the resonant frequency element into a ringing mode which thereby creates a false indication of a code tone.